The present invention relates to a circuit producing an output signal free from a leakage between output and inputs ends, and more particularly to a coefficient multiplying circuit which multiplies an input signal by a predetermined coefficient defined by the capacitance ratio of capacitors.
A conventional circuit multiplying an input signal by a predetermined coefficient consists of an input terminal, a first capacitor connected to the input terminal through a first switch, a second capacitor connected in series with the first capacitor, an operational amplifier having an inverting input port and an output port which are connected to the terminals of the second capacitor, respectively, and a non-inverting input port which is connected to a point of a reference voltage or a power supply voltage, an output terminal connected to the output port of the operational amplifier, a second switch connected in parallel with the second capacitor, and a third switch connected between the point of a reference voltage or a power supply voltage and the electrode on the input terminal side of the first capacitor. The first, second and third switches are usually transistors able to operate at high speed. In particular, MOS field effect transistors are most favorable because of their high resistances when nonconductive.
Even MOS field effect transistors, however, do not have an infinite resistance when nonconductive. Therefore, the electric charges stored in the first and second capacitors discharge through the switches. In other words, the output obtained deviates from the value to be produced by multiplying the input voltage by the capacitance ratio of the first and second capacitors, so that a correct output is not obtained.